Because there are more and more people wanting to take up flyfishing every day. Aside from the obvious scenario where someone who has fished a run for years suddenly finds himself crowded out, there is an increasing pressure on the fish as well.
And one other thing...while I have your attention...back in the day, esp. on rivers such as the North Umpqua, there was an unspoken code of conduct that everyone accepted and adhered to. Low-holing was not only frowned upon...people just didn't do it. They respected the other guy's claim on the stretch. At the same time a person didn't hog that stretch. They fished through and everyone got their turn.
The watchword was "respect"--for the fish and for other fishermen.
Nowadays with the increased pressure and people who haven't grown up with and/or don't share that kind of innate courtesy and respect, some of the joy and relaxation that was always there has dissipated. I've been on the D and seen people standing in runs...in the middle of the day...not fishing, just holding the run so that no one else can get in.
If I had a run or a pool that I had spent years learning and come to love, I wouldn't share it with strangers either. The fact that your mentor shared with you is a sign of trust. A wisdom passed on. When the day comes that you understand why the admonition was given, you'll be ready to pass your favourite places on to the next generation.
